Oncornaviruses are of current interest because they cause tumors in mice, chickens, and cats, and oncornavirus-like particles have occasionally been found in materials derived from human tumors. Studies with feline oncornaviruses are important because cats are outbred like man, not specifically inbred like mice and chickens. Cats thus represent the only free-living random-bred species where oncornaviruses have been shown to cause tumors. The feline oncornaviruses will replicate in cells from and cause tumors in many species, including monkeys. Antigenic changes of cell membranes provide an important tool for studying tumors and transformed cells, and such antigens have been used extensively in SV40 and Epstein-Barr virus systems. We looked for, and found such an antigen which was specific for feline oncornavirus infection or transformation. We showed that the antigen is expressed in all species or cells exposed and is common for all subtypes of feline oncornaviruses but not for mouse or avian viruses. This proposal concentrates on the nature of this antigen which I call the feline oncornavirus associated cell membrane antigen or FOCMA. The issues to be studied concern the relationship of FOCMA expression to the production of internal virus structural proteins (group-specific or gs), virion envelope antigens, and release of whole virus. It is important to know if FOCMA expression is an early virus function. If so, it might be a marker for cells that become transformed but only "defectively" infected-the cells and tumors that don't produce virus. Our previous studies showed that FOCMA is a valuable marker for studying the natural history of feline oncornaviruses, and the immune response to FOCMA apparently determines whether virus-infected cats get progressive tumors or recover and eliminate the virus. For this reason it is also important to know whether FOCMA immunity and virus neutralization are overlapping or distinct.